1. Field of the Invention
The present invention relates to a turbo molecular pump used, for instance, as a vacuum device for a semiconductor manufacturing apparatus.
2. Description of Related Art
A turbo molecular pump is designed such that rotor vanes attached to a rotor shaft rotating at high velocity and stator vanes fixed by an outer casing are alternately arranged so that a pair of the rotor vane and the stator vanes are arranged in multi-stages in an axial direction of the rotor axis. The rotor vanes include a plurality of blades each inclined at a predetermined angle. The stator vane includes a plurality of blades similarly to the rotor vane, and the inclined direction of each blade is opposite from the inclined direction of the blade provided in the rotor vane.
An axial interval between the rotor vane and the stator vane is determined from a viewpoint of convenience in design and so on. For example, the axial interval between the rotor vane and the stator vane located close to a inlet port is set to about 5 mm.
In the turbo molecular pump thus constructed, the rotor vanes are rotated by the rotation of the rotor shaft, and the gas molecular is beaten in the rotating direction so that the blades of rotor vanes are moved axially, thereby carrying out the discharge of the gas.
The turbo molecular pump of this type is used, for instance, for the purpose of the discharge for a vacuum chamber of the semiconductor manufacturing apparatus. That is, in order to carry out processing of the semiconductor in the vacuum chamber, it is necessary to always supply gas to the vacuum chamber, and discharge the gas supplied thereto by the turbo molecular pump.
However, recent tendency is directed toward the increase in gas amount to be supplied to the vacuum chamber, and also, the gas amount to be discharged by the turbo molecular pump in a normal operation is increasing.
A test in throughput characteristic was conducted to confirm whether or not a conventional turbo molecular pump can provide sufficient vacuum property (discharge performance) in case where the amount of the gas to be discharged therefrom is increased. The test result is shown-by one-dotted chain line A in FIG. 4.
Here, the gas to be discharged was nitrogen (N2), and a dry pump of 1300(1/min) was used as an auxiliary pump.
From the test result, it was found that the sufficient vacuum property was obtained (for example, the pressure of not more than 10xe2x88x922 Torr, i.e. 10 mTorr was obtained) in the case where the throughput of the gas to be discharged was small, but the sufficient vacuum property could not be obtained (for example, the pressure was larger than 10 mTorr) in the case where the throughput of the gas to be discharged was large.
Based on this result, the present inventor has made the detailed study to seek a cause or reason why the sufficient vacuum property could not be obtained in the case where the throughput of the gas was large, and discovered and obtained a novel view that the lowered discharge performance was caused by a fact that the discharged gas did not form molecular flow in an axial space between the rotor vanes and stator vanes near the inlet port of the turbo molecular pump. In other words, the present inventor has obtained such a novel view that the lowering in the vacuum property in association with the increased throughput of the gas was closely related to the axial interval between the rotor vanes and the stator vanes.
The present invention was made on the basis of the above-described novel view, and an object of the present invention is to provide a turbo molecular pump capable of ensuring the sufficient vacuum property while maintaining the increased throughput of the gas even when the throughput of the gas to be discharged in a normal operation is increased.
To attain the above-described object, a turbo molecular pump according to the present invention is characterized by comprising:
a rotor shaft;
a bearing for rotatably supporting the rotor shaft;
a motor for rotating the rotor shaft supported by the bearing;
rotor vanes of multiple stages provided to the rotor shaft; and
stator vanes of multiple stages arranged between the rotor vanes of multiple stages, respectively;
wherein that an axial interval between at least one of the rotor vanes and a corresponding one of the stator vane is set to a value by which a gas can be dealt as a molecular flow under a condition of pressure not less than 10 mTorr during a normal operation.
The present invention can be also expressed as follows:
That is, a turbo molecular pump according to the present invention is characterized by comprising:
a rotor shaft;
a bearing for rotatably supporting the rotor shaft;
a motor for rotating the rotor shaft supported by the bearing;
rotor vanes of multiple stages provided to the rotor shaft; and
stator vanes of multiple stages arranged between the rotor vanes of multiple stages, respectively;
wherein at least one axial interval between one of the rotor vanes and a corresponding one of the stator vane is set to a value by which a gas can be dealt as a molecular flow under a condition that a discharge throughput during a normal operation is not less than 1000 SCCM.
The one of rotor vane is located closest to a inlet port among the rotor vanes of multiple stages, and the corresponding one of the stator vane is located closest to the inlet port among the stator vanes of multiple stages.
The at least one axial interval between one of the rotor vanes and a corresponding one of the stator vane is set based on a mean free path of molecular gas.
As described above, according to the present invention, the axial interval between the stator vane and the rotor vane is set to such a value as to be capable of dealing with the gas as the molecular flow under the condition that the pressure in the inlet port is equal to or more than 10 mTorr during the normal operation.
Therefore, according to the present invention, the gas can be dealt as the molecular flow under the condition that the pressure is equal to or more than 10 mTorr during the normal operation, and sufficient discharge performance can be obtained. Thus, even if the throughput of the gas supplied to the vacuum chamber during the normal operation is increased compared with the conventional one, the present invention can ensure the required pressure (required vacuum property) while maintaining the increased throughput of the gas.